7. DETAILS OF NEW M87 PNLF STUDIES

Bottinelli et al. (1991)
and Tammann (1993)
argued that PNLF distances to Virgo
ellipticals were underestimated because the luminosity depth of the PN
surveys was not adequate to sample beyond the brightest (0.5 mag) edge
of the PNLF. Since that edge is nearly linear in the logarithmic PNLF,
the method becomes insensitive to distance
modulus. In addition, those authors challenged the PNLF distances on the
basis that a shallow survey of a large galaxy will suffer from a sample
size bias. The sense of this argument is that N objects are more
likely to be drawn from the low probability bright
tail of the large elliptical galaxy PN sample than are N objects
from the smaller sample in M31's bulge.

While it is true that PN surveys must extend deep enough to sense the
curvature of the PNLF reliably with statistical methods, the required
depth is only 0.8 mag. With the exception of NGC 4649 which was observed
under poor conditions,
Jacoby, Ciardullo, & Ford
(1990)
estimated the depth of their surveys for 6 Virgo galaxies to be ~ 1.0
mag. Thus, it seemed unlikely that a serious systematic error was
contaminating those distances.

An independent assessment of the likelihood of a serious systematic
error is provided by recent Cepheid distances to Virgo galaxies. The
average PNLF distance to Virgo,
based on 6 galaxies, is 15.3 Mpc (using the modern M31 distance and
extinction for the zero-point). This result agrees very well with the
Ferrarese et al. (1996)
distance of 15.8 Mpc to M100 based on HST Cepheids, and the
Pierce et al. (1994)
distance of 14.7 Mpc to NGC 4571 based on CFHT Cepheids. In addition,
Sandage et al. (1996)
reports HST Cepheid distances to three near-Virgo galaxies: NGC 4496 at
16.6 Mpc, NGC 4536 at
16.6 Mpc, and NGC 4639 at 25.1 Mpc. Thus, four galaxies are
reported in
the range 14.7 to 16.6 Mpc, and these are very comparable to the PNLF
range of distances (14.3 to 16.2 Mpc). One galaxy, though, is behind all
of these. It is unclear which, if any, of
these spirals represents the distances to the ellipticals, but it is
evident that most of the
spirals (four out of five) have distances that support the PNLF distances.

A direct resolution of the challenges to the PNLF distances lies in a
short observing
project. Deep PN observations in M87 can push well into the plateau
region of the PNLF. Data were obtained in April 1995 with the KPNO 4-m
telescope to examine the claim that the earlier Virgo data were not deep
enough. A total of 7 hours of on-line
integration were devoted to detecting fainter PN. This survey also
extends to large radial (10 arcmin) distances from M87's nucleus. The
survey results are described below.

Figure 3 shows the new PNLF for M87. A total of
320 PN were identified, but many are
fainter than the completeness limit. A total of 201 PN are in the
complete sample which extends ~ 1.2 mag down the PNLF.

Figure 3. The 1995 PNLF for M87 based on 7
hours at the KPNO 4-m. This PNLF extends
1.2 mag down the PNLF and clearly reaches beyond the linear portion of
the bright edge
(25.9 < m5007 < 26.3) of the PNLF. The scaled and
shifted PNLF from M31 is superposed
to illustrate the agreement with the reference galaxy's PNLF and
demonstrates that the M87
PNLF is not a power law over this regime. Note the single very luminous
object at m5007 = 25.6
which was found first in the 1990 survey. See
Jacoby et al. (1996)
for a discussion of what this luminous object may be.

Figure 4 shows a curious effect, though, which
has not been fully evaluated at the
time of this conference. That is, when the sample of PN is divided in
half such that the PN drawn from M87's halo are separated, the PNLF for
the inner half (out to 4 arcmin in radius) are systematically fainter by
0.3 mag than the halo sample. I return
to this point below, but note here that the more reliable distance is
derived from the inner sample because it is less likely to be
contaminated by intracluster PN. That
distance, 14.4 ± 1.3 Mpc (on the modern M31 distance scale), is
nearly identical to the
Jacoby, Ciardullo, & Ford
(1990)
value of 14.9 ± 1.2 Mpc.

Figure 4. The 1995 PNLF for M87 where solid
points represent those PN found in the inner
4 arcmin and open circles show those PN found beyond 4 arcmin and out
to 10 arcmin.

Thus, a deeper PNLF argues against the contentions of
Bottinelli et al. (1991)
and Tammann (1993)
that the PNLF distance to Virgo is underestimated as a consequence
of inadequate survey depth.

With the new larger sample of PN, it is possible to investigate the
effects that different
sample sizes have on the final PNLF distance. Subsets of PN were drawn
randomly from the sample of 201 PN. Distances for these subsamples of
20, 30, 40, 50, and 100 PN were
derived following our standard procedures and compared to the distance
based on 201 PN. Figure 5 shows the magnitude of
the effect of sample size differences.

Figure 5. Results of a Monte Carlo
experiment to estimate the effect of deriving distances with
different sample sizes. The error bars represent the scatter in the
multiple attempts to derive distances with a given number of PN in the
sample.

In the worst case, for a sample of 20 PN, there is a slight tendency to
overestimate the distance to a galaxy by up to 3%. Again, this
contradicts the challenges of
Bottinelli et al. (1991)
and Tammann (1993)
who claim that our distances would be
underestimated. The reason that the effect is small is that the
statistical process described by
Ciardullo et al. (1989)
is cognizant of the sample size and adjusts the derived
distances to the most likely one for a given sample. That is, a
statistical correction for sample size has always been applied to the
PNLF results.

As noted above, M87's halo PNLF is ~ 0.3 mag brighter than the central
PNLF. We can all agree that the halo of M87 is not 15% closer
than its core! Thus, something must
be artificially enhancing the brightness of the PN in the outer
regions. Since we have
not seen this effect in the two other samples that permit a similar
radial test (Cen A,
Hui et al. 1993;
NGC 4494,
Jacoby, Ciardullo, &
Harris (1996)),
we consider what could
cause such an effect here. Five possibilities come to mind:

Metallicity decrease in halo

Age decrease in halo

Dust near center

Instrumental effect

Intracluster PN contamination

The color gradient in ellipticals is such that the outer halos are bluer
than the inner
regions. The first 3 possibilities above have been suggested as possible
causes of gradients.

The first, a metallicity decrease, reduces the luminosity of PN, in
contradiction to the observed effect. The second, the presence of a
young population can enhance the PNLF
luminosity if the ages of the halo stars are < 0.5 Gyrs, provided the
central population is > 3 Gyrs. We can neither dismiss nor confirm
this possibility.

Although the effect has not been seen before, a brightness enhancement
in the halo could be caused by instrumental effects. Thus far,
flat-fielding errors and filter transmission variations have been dismissed.

The fifth option initially seems highly speculative. The key point is
that a large population of intracluster stars exists and can produce PN
having a range of distances
representing the full depth extent of the Virgo cluster. Thus, some PN
may be foreground to M87 and appear brighter than M87's own PN, while
other PN will be in the background
and be lost in the faint end of the PNLF. Since the number of
intracluster PN found is
proportional to the area surveyed, there is a survey bias against
finding foreground PN in the smaller central region, while
simultaneously, there is a bias in favor of finding true
M87 PN at the center where the stellar density is high.

What is the likelihood of finding intracluster PN?
Arnabaldi et al. (1996)
found that 3 of 19 PN in the
Jacoby, Ciardullo, & Ford
(1990)NGC 4406 sample are intracluster.
Since that sample is biased against intracluster PN due to velocity
rejection in the survey filter (NGC 4406 was sampled at its systemic
velocity of -220 km/s, or about 1500 km/s from the Virgo systemic
velocity), there are likely to be many intracluster PN.
Arnabaldi et al. (1996)
discuss the intracluster population in detail. To zeroth order,
the number density of intracluster PN is not a problem.

A definitive source of the enhanced halo PNLF in M87 is not possible at
this time. Kinematics can be used to investigate the likelihood that
intracluster PN are contaminating
the halo sample. The other likely causes, extinction and very young
populations, seem less secure at this time because we don't know that
they exist while we do know that intracluster PN do exist.